Field of the Invention
The present invention relates to a laser processing apparatus for performing laser processing to a workpiece such as a semiconductor wafer.
Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby define a plurality of separate regions where a plurality of devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the division lines to thereby divide the regions where the devices are formed from each other, thus obtaining a plurality of individual semiconductor chips.
As a method of dividing such a semiconductor wafer along the division lines, there has been tried a laser processing method using a pulsed laser beam having a transmission wavelength to the wafer to apply the pulsed laser beam to the wafer in the condition where the focal point of the pulsed laser beam is set inside the wafer in its subject area to be divided. In such a dividing method using this laser processing method, a pulsed laser beam having a transmission wavelength to the wafer is applied to the wafer from one side thereof in the condition where the focal point of the pulsed laser beam is set inside the wafer, thereby continuously forming a modified layer inside the wafer along each division line. Thereafter, an external force is applied to the wafer along each division line where the modified layer is formed to be reduced in strength, thereby dividing the wafer along each division line.
As another method of dividing a platelike workpiece such as a semiconductor wafer along the division lines, there has been proposed a method of applying a pulsed laser beam having an absorption wavelength to the workpiece along the division lines to thereby form a laser processed groove on the workpiece along each division line by ablation and next breaking the workpiece along each laser processed groove by using a mechanical breaking apparatus.
Such laser processing is performed by using a laser processing apparatus including a holding table for holding a workpiece, laser beam applying means for applying a laser beam to the workpiece held on the holding table, and feeding means for relatively moving the holding table and the laser beam applying means in a feeding direction. In such a laser processing apparatus, there is a problem such that when the laser beam overruns the workpiece held on the holding table, the upper surface of the holding table may be damaged by the laser beam.
To solve this problem, there has been proposed a laser processing apparatus including detecting means for applying detection light to a workpiece held on a holding table and detecting whether or not the workpiece is present at a laser beam applying position according to the intensity of return light reflected from the workpiece, wherein when the detecting means detects that the workpiece held on the holding table is not present at the laser beam applying position, the application of a laser beam from laser beam applying means is stopped (see Japanese Patent Laid-open No. 2010-127920, for example).